A large variety of articles are manufactured from polyolefin plastic through a variety of methods such as, for example, injection molding, rotational molding, blow molding, and/or thermoforming. Although molding various polyolefin plastic articles may be relatively straightforward, permanently decorating the articles is challenging. In general, polyethylene and polypropylene plastics are non-polar, and resist adhesive coatings that could be used to mark the articles with decorative indicia.
Attempts to improve adhesion of labels to untreated plastic surfaces have included the use of heat activated labels, such as disclosed in U.S. Pat. No. 7,622,171. These labels are applied to the plastic articles in the form of transfers having an adhesive layer comprising a vinyl acetate resin, a tackifying resin, and a microcrystalline wax. However, this approach adhesively bonds the label only to the outer surface of the article. In other words, the label applied in this way is not part of the plastic, not integral to the article, but instead is attached on the surface of the plastic article where the label becomes subject to wear and eventual delamination.
U.S. Pat. Nos. 8,349,917 and 9,296,243 teach a thermal printing ink transfer made by fusing a polyethylene-based ink onto a carrier sheet or film. Transferring the indicia from the thermal printing ink transfer to a surface of a molded plastic article comprises heating the ink transfer to a temperature of about 450° F. (232° C.) to about 650° F. (343° C.) and compressing the transfer against the plastic article at a pressure of about 500 psig (3.45 MPa) to about 1000 psig (6.9 MPa). The heating and compressing are accomplished, for example, by forcing a heated flat metal plate (i.e., a heated platen) having a hard and thin rubber die against the article, while the printing ink transfer is indexed into position between the platen and the article. With this process, indicia are fused into the surface of the plastic article, becoming integral with the article.
Although the process disclosed in the '917 and '243 patents may be effective in decorating flat surfaces of plastic articles capable of withstanding the high pressure of the platen, the process is typically ineffective for decorating a surface that is not flat, or for decorating a surface of a plastic article not capable of withstanding the pressure of the platen (e.g., a plastic article having a hollow structure, or raised texturing, or other structure that would be readily crushed or damaged by the pressure of the platen). The heated press assembly comprising the platen and the hard rubber die is entirely inflexible and is incapable of conforming to uneven surfaces.
In view of these deficiencies, new methods, apparatus and systems for transferring indicia onto uneven surfaces and onto flat or uneven surfaces of fragile plastic articles are still needed.